Flood hazards in Wisconsin

Science Center Objects

A summary of USGS resources and data related to flooding hazards in Wisconsin.

Example graph of current stream stage compared to historical peaks

Example graph showing current stream stage (far left), highest stage in the past year (second from left), and the four highest historical peak stage measurements (four dark-blue bars on right). This chart also shows the National Weather Service flood stage for this site (red line).

What is a flood?

A flood occurs when relatively high water overflows the natural or artificial banks of a stream or coastal area and submerges land not normally below water level.

Why do floods occur?

In Wisconsin, floods usually are caused by one of three scenarios:

  • In the summer, thunderstorms associated with slow-moving frontal systems
  • In the winter, floods can occur when rain falls on snow causing rapid snowmelt
  • Ice-jam floods are also a hazard during the winter

 

Wisconsin hydroclimatology

The principal moisture-bearing air masses for Wisconsin originate in the Gulf of Mexico (moisture can also come from air masses originating in the Pacific Ocean, but these lose most of their moisture crossing the Rocky Mountains). Lake-effect precipitation is a minor source of moisture for Wisconsin; lake-effect precipitation is most noticeable in the early winter when the cold air blowing over the unfrozen Great Lakes absorbs moisture, which then is precipitated as snow on the lake shores.

Factors that contribute to flood events in Wisconsin streams include:

  • drainage area
  • storage (the percentage of drainage area covered by lakes, ponds, swamps, and wetlands)
  • heavy rainfall, snowfall, or snowmelt
  • soil permeability of the least-permeable soil horizon
  • forest and vegetation cover (the percentage of drainage area covered)

In urban areas, factors contributing to flooding include:

  • drainage area
  • percentage of drainage area that is impervious (parking lots, roads, rooftops)

 

Figure of hydrographs from June 2008 floods in Wisconsin

Hydrographs showing selected USGS streamgages in southern Wisconsin during the June 2008 floods (from SIR 2008-5235).

USGS flood data

Real-time and historical information on floods in Wisconsin is available through the USGS National Water Information System (NWIS).

A map of Wisconsin gages currently at high-flow or flood conditions can be found here. Click on a site to get a summary of conditions, view the current hydrograph, or see a chart of how the current stage compares with historical peak flows and floods. Animations of current and historical high-flow and flood events (by date) are also available.

A table of current discharge measurements and historical peaks for all real-time streamgages in Wisconsin is also available. You can use the USGS WaterAlert service to receive an email or text alert if a particular streamgage exceeds a user-specified gage height or discharge value.

 

How the USGS contributes to flood response

During a flood, the USGS and the National Weather Service work in concert with its own precipitation data to forecast river stages and flow conditions on large rivers and their associated tributaries. The USGS collects streamflow data, the NWS collects precipitation data and combines both types of data to make flood forecasts. The USGS will continue to measure the actual height of the flood during the event.

During large, regional flood events, the USGS will deploy field crews to install temporary rapid-deployment streamgages and sensors ahead of the storm to track flooding as it happens. Some of these sensors will relay real-time information to emergency managers during the flood event, while others will be retrieved by the field crews as soon as floodwaters have receded. These hydrographers will also look for physical evidence of high-water marks left behind by the flood. Post-flood analyses allow USGS scientists determine the extent of the flood and help communities prepare for future flood events.